I. Field of the Invention
The present invention relates to an improved process for catalytically cracking a naphtha hydrocarbon feed to produce light olefins products, such as ethylene. propylene and butylene, and a relatively less aromatics. The process includes contacting a naphtha hydrocarbon feed with a pentasil zeolite catalyst which contains phosphorus and a promoter metal. The present invention also relates to a catalyst composition useful for manufacturing light olefins from a naphtha hydrocarbon feed.
II. Description of the Prior Art
Production of light olefins, such as ethylene and propylene, is a major industrial process and forms a backbone of the chemical industry. Conventionally, naphtha feedstocks containing paraffinic and isoparaffinic compounds, naphthenes and aromatics are thermally cracked to produce light olefins. The naphtha thermal cracking process of choice has traditionally been steam pyrolysis, usually performed by exposing naphtha and steam to relatively high temperatures in the tubular coils of a fired heater.
Examples of previously known olefin processes include those described in U.S. Pat. Nos. 5,446,229 and 5,254,781. Although steam pyrolysis is widely practiced, this process is energy intensive, not very selective, produces coke and releases significant amounts of carbon dioxide into the air. Accordingly, chemical manufacturers have long recognized a need for an alternative naphtha cracking process.
One alternative to steam pyrolysis process is a naphtha catalytic cracking process, in which a hydrocarbon feed is contacted with a catalyst and the hydrocarbon is converted selectively to an olefin-containing mixture. Although various naphtha catalytic cracking processes have been previously proposed, none has been adopted by the chemical manufacturing industry on a commercial scale. Apparently, previous naphtha catalytic cracking processes do not produce commercially important light olefins with sufficient selectivity or yield, and typically produce undesirable amounts of methane and aromatics as unwanted byproducts. In contrast, a practical and economic naphtha catalytic cracking process will selectively produce ethylene and propylene in abundance, while producing minimal amounts of methane and aromatics.
The process of the present invention employs a pentasil zeolite catalyst system which includes phosphorus together with certain promoter metals. Although pentasil zeolite materials, such as zeolite ZSM-5, have been widely publicized as hydrocarbon conversion catalysts, the present invention is directed to specific uses of a pentasil zeolite catalyst system for selectively cracking naphtha to produce light olefins. Examples of previous catalyst systems include U.S. Pat. No. 3,972,832; U.S. Pat. No. 4,044,065; and U.S. Pat. No. 5,171,921.
One aspect of the present invention is to convert a hydrocarbon stream such as, for example, a naphtha stream, to a stream including light olefins, such as ethylene and propylene. Another aspect of the invention is to convert a light hydrocarbon stream including C5 to C7 paraffins and isoparaffins to an olefin stream which includes ethylene and propylene. Still another aspect of this invention is to convert a hydrocarbon stream to olefins, without coproducing a significant amount of methane and aromatics. Other aspects of the invention may be present in the further description and examples.
The invention is a naphtha cracking catalyst and an improved catalytic naphtha cracking process for producing olefins from paraffins, particularly paraffins which are present in a hydrocarbon mixture commonly known as naphtha. The process provides relatively higher propylene yields and significantly lower methane yields over the commercially important range of about 60 to about 90 percent naphtha conversion, while providing about the same or lower yields of aromatics and light paraffins over the range, as compared to well known prior art catalytic and thermal processes. Additionally, the process resists deactivation of the catalyst by coking.
In one aspect, the invention is a process for producing a relatively light olefin. Naphtha is the preferred feedstock for this process. In the process, a hydrocarbon feedstock, which includes hydrocarbons having about three to about twenty carbon atoms, preferably a paraffinic and isoparaffinic hydrocarbons having about four to about eleven carbon atoms per molecule, is passed into a reactor containing a pentasil zeolite catalyst. The zeolite-containing catalyst typically includes about 0.1 to about 10 weight percent phosphorus and about 0.1 to about 10 weight percent of a promoter metal selected from the group consisting of gallium, germanium, tin and mixtures thereof. At least a portion of the hydrocarbon is converted to produce an olefin having about two to about three carbon atoms per molecule. The hydrocarbon may be passed into the reactor together with a heat-conducting diluent such as steam, nitrogen, alkanes such as methane and ethane, and mixtures thereof which are substantially inert under the process conditions used.
In another aspect, the invention is a process for producing an olefin, which process comprises contacting a naphtha which includes a paraffin having about four to about eleven carbon atoms per molecule with a catalyst in a reactor at a temperature of about 400 to about 650 degrees C. and a pressure of about one to about three atmospheres. The catalyst is a pentasil zeolite catalyst having a silicon to aluminum atomic ratio of about 10 to about 400 and On which is typically placed about 0.1 to about 10 weight percent phosphorus and about 0.1 to about 10 weight percent of a promoting metal selected from the group consisting of germanium, gallium, tin, and mixtures thereof. At least a portion of the naphtha is cracked in the reactor to produce olefins having about two to about three carbon atoms per molecule. The naphtha may be passed into the reactor together with a diluent in the molar ratio in the range of about 9 to about 0.1. Alternatively, the naphtha may be passed into the reactor together with additional propane in a molar ration in the range of about 6 to about 1. A portion of the reactor product may be recycled to the reactor.
In yet another aspect, the invention is a catalyst useful for producing light olefins from a hydrocarbon mixture, such as a naphtha, which includes one of more paraffins. The catalyst comprises a pentasil zeolite comprising silicon and aluminum in a silicon to aluminum atomic ratio of about 10 to about 400, about 0.1 to about 10 weight percent phosphorus, and about 0.1 to about 10 weight percent of a promoting metal selected from the group consisting of germanium, gallium, tin and mixtures thereof. Preferably, the pentasil zeolite is ZSM-5, and the weight ratio of phosphorus to promoter metal is in the range of about 1:1 to about 5:1.